Structure Of ATP

How ATP Stores Energy

Bonds between the phosphate groups are unstable and easily broken, releasing energy.

This is catalysed by the enzyme ATPase.

ATP + H20 < - > ADP + Pi + Energy

Addition of Pi to ADP is called phosphorylation; ATP transfers energy.

There are three forms of phosphorylation:
-Oxidative phosphorylation: which occurs on the membranes of mitochondria in aerobic respiration.
-Photophosphorylation: which occurs on the membranes of chloroplasts during photosynthesis.
-Substrate-level phosphorylation: which occurs when phosphate groups are transferred from donor molecules to ADP to make ATP.

The first two release ATP by the transfer of electrons along a chain.

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Importance And Role Of ATP

Instead of an uncontrolled release of energy like in glucose - which would destroy cells - ATP allows for small amounts of energy in steps when required.

ATP can be hydrolysed for when it is needed; ATP -> ADP + Pi + 30kJmol-1

Further advantages of ATP include:
-The hydrolysis is one reaction that releases immediate energy.
-Only one enzyme is required.
-A common source of energy throughout the cell in many chemical reactions, it is a universal molecule.

Respiration

Glycolysis

Initial stage in both aerobic and anaerobic respiration.

Happens as follows:
-Glucose is activated by addition of 2 ATP's, lowering activation energy.
-Glucose is converted to GC hexose phosphate, which splits into 2 triose phosphates.
-Hydrogen is removed from each of the two TP molecules, and forms reduced NAD.
-The TP is thus converted into pyruvate, these steps generate 4 ATP molecules by substrate-level phosphorylation - but 2 are used, producing a net of 2 ATP.
-2 molecules of red. NAD are also formed.

Energy still remains in pyruvate that can be released in Krebs Cycle.

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Link Reaction

Links glycolysis to Krebs cycle:
-Pyruvate diffuses from CYTOPLASM to MITOCHONDRIAL MATRIX.
-Pyruvate is then decarboxylated.
-It is also dehydrogenated, where the H is accepted by NAD.-The remaining molecule then combines with coenzyme A to form acetyl coenzyme A.

Pyruvate + NAD + CoA -> acetyl CoA + red. NAD + CO2

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Krebs Cycle

The function of Krebs cycle is to liberate energy from the C bonds to provide ATP/NADH.

This occurs by:
-Acetyl CoA enters Krebs and combines with a 4C acid, to form a 6C compound.
-The 6C undergoes reactions during which CO2 and H are removed, after which the remaining 4C compound is regerated to reform the original 4C acid.
-Two steps include decarboxylation, and four involve dehydrogenation.

So for each molecule of glucose:
-2 ATP are formed by substrate-level phosphorylation.
-6 NADH and 2 FADH2.
-2 molecules of CO2.

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Krebs Cycle Diagram

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Electron Transport Chain

A series of pumps and carriers, releasing energy as ATP in the INNER MEMBRANES.

CO2 is waste, but H are carried by coenzymes NAD/FAD and synthesise 3 or 2 ATP accordingly; for each red. NAD 3 ATP are formed, each red. FAD there are 2 formed.

This is decribed by the chemiosmotic theory:
-NADH donates the e- from the H to the electron carriers, and the H+ remains in solution.
-The e- provide energy to pump H+ into mitondrial inter-membrane space.
-The e- move along providing energy for each pump in turn.
-Eventually a concentration gradient is set up as the membrane is impermeable to H+.
-The channel where H+ can move through is associated with ATP synthetase.
-When H+ diffuse back into the matrix, kinetic energy causes the generation of ATP.
-At the end of the chain the e- and H+ combine with O2 to form H20.

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Anaerobic Respiration

If there is no O2 available then only gycolysis can take place.

For glycolysis to occur pyruvate and H must be constantly removed and NAD regenerated.

So pyruvate accepts the H from the NAD.

This all occurs in two ways:
-The first is in animals and occurs mainly in muscle tissues, during exercise O2 cannot be supplied enough and so pyruvate accepts H and converts into LACTATE.
-The second occurs in micro-organisms such as yeast; the pyruvate is first decarboxylated to produce ethanal which is then converted to ethanol with the addition of H from NAD.

Alternative Respiratory Substrates

Lipids:
-Fat provides an energy store and is used for when carbs are low.
-It's split into glycerol and fatty acids.
-The first is phosphorlyated with ATP, and dehydrogenated with NAD; and converted into TP.
-The second ones are formed into 2C fragments which enter Krebs as acetyl CoA.

Proteins:
-This can be used under extreme circumstances, when tissue protein is mobilised.
-It is hydrolysed into amino acids and deanimated in the liver, where the amino group forms urea and the residue forms acetyl CoA or pyruvate.